1. Field of the Invention
The present invention relates to a head position control method for a disk device for suppressing the displacement of the head by disturbance and the disk device, and more particularly to a head position control method for suppressing the displacement of the head by external vibration and the disk device.
2. Description of the Related Art
In the case of a disk device, which reads and writes data on a rotating storage medium (e.g. disk medium) by a head, the head is positioned on a desired track of the storage medium and data is read from/written to the track by the head. For such a disk device, such as a magnetic disk device or optical disk device, it is extremely important to accurately position the head on the target track for improving the recording density.
One factor that interrupts this positioning is eccentricity, which is caused by the center of the servo signals disposed concentrically on the disk that is different from the rotation center of the motor. A technology to correct this eccentricity is a control method using an observer (e.g. Japanese Patent No. 3,460,795).
Eccentricity generates sinusoidal positional fluctuation synchronizing with an integral multiple of the rotation frequency. The above-mentioned observer control method can suppress such sinusoidal positional fluctuation so as to position the head accurately on the target track. However for this eccentricity correction, the frequency to be corrected must be known in advance. For example, the frequency must be an integral multiple, multiplied once or twice, of the rotation frequency.
The second factor that interrupts this positioning is vibration, which is applied to the disk device from the outside. This vibration has various waveforms, but here a method for handling sinusoidal vibration will be described. By applying the above-mentioned eccentricity correction control, it becomes possible to handle a frequency which is not an integral multiple of the rotation frequency.
In the case of the above-mentioned conventional configuration, it is based on the assumption that the frequency of the disturbance is known in advance. The external vibration to be applied, however, is unknown at the point of designing the control system, so it is impossible to know the frequency in advance. Therefore some means of detecting this unknown frequency is necessary, and only if the frequency can be detected, the positional fluctuation by external vibration can be suppressed using such a control method as the above-mentioned Japanese Patent No. 3,460,795.
FIG. 44 is a block diagram depicting a conventional control system for detecting the disturbance frequency and suppressing sinusoidal disturbance with a predetermined frequency. The position error ‘e’ between the target position ‘r’ and the observed position ‘y’ is calculated by the computing unit 100, and is input to the controller 102 (Cn) for performing feedback control. The controller 102 outputs the control current value Un by known PID control, PI control+LeadLag, and observer control.
A frequency estimation unit (ω estimation) 106 for estimating disturbance, and a compensator (Cd) 104 for suppressing disturbance of a predetermined frequency by adaptive control are added to this controller 102. The sum of the output Un of the controller 102 (Cn) and the output Ud of the compensator 104 (Cd) that is U, is supplied to the control target 103 (P). This frequency estimation unit 106 estimates the angular frequency ω (=2πf) of the disturbance based on the position error ‘e’, and introduces it to the transfer function of the disturbance frequency suppression of the compensator 104. The compensator 104 calculates the recurrence formula (adaptive control formula) of the sine wave from the position error ‘e’ and the estimated angular frequency ‘ω’, and calculates the compensating current output Ud.
In this way, the frequency of disturbance is detected and the unknown frequency is suppressed so that conventional eccentricity correction control can handle the disturbance of an unknown frequency in a certain range (e.g. Proceedings of the 40th IEEE Conference on Decision and Control, pp. 4909-4914, issued December 2001). As a method of estimating an unknown frequency and suppressing the disturbance of an unknown frequency, assuming the recurrence formula of the sine wave (e.g. The Institute of Electrical Engineers of Japan, Technical Meeting on Industrial Instrumentation and Control, IIC-04-70, “Frequency following type peak filter”, issued Sep. 10, 2004), or correcting the drive amount of the control target by introducing an adaptive rule based on the above-mentioned error signal (e.g. Proceedings of the 40th IEEE Conference on Decision and Control, pp. 4909-4914, issued December 2001), have been proposed. Another proposal is estimating an unknown frequency by the error signal, generating a disturbance suppression signal in the position level, correcting the error signal and inputting it to the controller (e.g. U.S. Pat. No. 6,762,902).
For example, as an example of the open loop characteristics of the system in FIG. 45 show, the zero cross frequency of the open loop characteristics is set to 1000 Hz, and the target disturbance frequency is set to 500 Hz. This is the characteristic expressed by (Cn+Cd) P in FIG. 46. By setting the gain of the open loop characteristics of a specified frequency (500 Hz in this case) to high in this way, suppression of disturbance can be implemented.
Recently such a disk device, in particular a hard disk drive (HDD), is installed in mobile equipment, such as a portable personal computer, portable terminal, portable telephone and portable AV equipment. When the disk device is used in this environment the disturbance frequency is unknown and multiple, so the disk device must adapt to a wide range of disturbance frequencies.
Therefore if a conventional compensator for suppressing external vibration is attached to a conventional controller, a coefficient, according to the frequency of the external vibration, must be appropriately set in the suppression signal generation formula. In the case of the system in FIG. 44, the gain and the phase of the transfer function (adaptive rule) of the compensator 104 for suppressing external vibration must be adjusted. If this adjustment is not appropriate, the control system becomes unstable.
The above-mentioned various conventional disturbance suppression control methods determine such coefficients as the gain and the phase in advance based on experience or on estimate using the design theory of the feedback system, and the gain and the phase are fixed values in a generation formula. In such a setting of coefficients, disturbance suppression by a disturbance frequency in a certain range (e.g. estimated frequency range of the servo system) is possible.
However if the disk device is used in the above-mentioned mobile environment, the range of the unknown disturbance frequency changes depending on the operating conditions, and cannot be easily estimated. In the case of the adaptive control of a disturbance in prior art, the frequency range that can be followed up is limited, so in the above-mentioned operation environment, an adjusted gain and phase may be inappropriate, and may make the control system unstable. The level of disturbance cannot be specified either, which makes the control system unstable, and may amplify errors or may even diverge errors.